臺灣博碩士論文加值系統

本論文由兩個主題所組成.第一個主題是研究矽(111)-7x7表面在高溫
時與微量的氧分子的反應情形.第二個主題是研究矽(111)-7x7表面固有的
點缺陷在高溫的動態行為.在第一個主題中,我們使用可變溫的隧穿式掃描
顯微儀(Scanning Tunneling Microscope: STM)研究矽(111)-7x7表面與
微量氧氣的反應情形.大多數的氧分子吸附在最表層的矽原子上,在 STM
的影像上呈現為亮點.當溫度超過250 C 時,吸附的氧分子開始在7x7的半
個單位晶格內作最近鄰的跳躍.分析數個溫度的跳動頻率,可求得氧分子的
跳躍活化能(activation energy)及頻率常數(frequency factor). 我門
同時發現在氧分子的跳躍過程中,有兩個中間態存在.只有一定比例的跳躍
事件可以觀察到這兩個中間態.同樣地分析數個溫度的跳躍事件,我門近一
步求得在跳躍的過程中,各分子態與中間態的跳躍活化能及頻率常數.我們
提出一個氧分子的跳躍模型,可以圓滿地解釋氧分子的跳躍行為.在第二個
主題,我們的研究對象轉向某一種存在矽(111)-7x7表面上的固有點缺陷(
point defect),這種點缺陷在STM的影像上呈現為黑點且在溫度超過 500
C 後,開始在表面上跳動.經我們的確認,這種點缺陷並不是表面上的空位(
vacancy),也不是由摻雜物(dopants)所造成的.我們更進一步地證實這種
點缺陷也不是由超高真空(UHV)腔內的主要殘存氣體所形成的.

This thesis is composed of two parts. The first subject is
about the reaction of Si(111)-7x7 surfaces with low-exposure
oxygen. In the second part, we study a special type of intrinsic
point defects of Si(111)-7x7 surfaces. In the first topic, we
use a variable-temperature scanning tunneling microscope (STM)
to study the reaction products after exposing the Si(111)-7x7
surface to low-dose oxygen. Most of the oxygen molecules adsorb
on top of Si adatoms and appear as bright sites. The adsorbed
oxygen molecules start to jump between adjacent adatom sites
within a half of the 7x7 unit cell when temperature exceeds
about 250 C. From Arrhenius plots, the activation energies and
frequency factors are determined. We also resolve two short-
lived intermediate states which mediate the B site hopping. We
derive all jump barrier heights between molecular state and
intermediate states. A model is proposed which can explain the
hopping process well. In the second topic, we use an STM to
observe the defects on Si(111)-7x7 surfaces above 500 C. We find
a special type of point defects which appear darker than Si
adatoms at tunneling biases ranging from -3.0 to +3.0 V.These
defects survive even after flashing the sample up to 1250 C. We
name these point defects pseudo-vacancies (PVs) because they are
not real vacancies. We also verify that PVs are not induced by
adsorption of major residual gases in the ultrahigh vacuum (UHV)
chamber, nor by dopants. We observe PVs hop between nearest
neighboring Si adatom sites at temperatures below 610 C. Most
jumps are within a half of the 7x7 unit cell. Similarly, after
analyzing thousands of STM images recorded from 522 to 607 C, we
determine the activation energies ranging from 1.66 to 2.27 eV
and frequency factors from 10^7.7 to 10^11.7 Hz. Varying the
tunneling currents produce almost no effects on the hopping, but
varying the scanning speeds produces a small effect.

Cover
Acknowledgments
Abstract
Contents
List of Figures
Chapter 1. Introduction
Chapter 2. Si(lll)-7x7 Surfaces and Operating Procedure
2.1 Si(lll)-7x7 Surfaces
2.2 Apparatus
2.2.1 Vacuum System
2.2.2 Scanning Tunneling Microscope
2.3 Operating Procedure
2.3.1 Sample and Tip Preparation
2.3.2 Temperature Calibration
Chapter 3. Low Oxygen Exposure on Si(lll)-7x7 Surface
3.1 Introduction
3.2 Experimental Method
3.3 Results and Discussion
3.3.1 Site Hopping of 02 Molecule
3.3.2 Intermediate States in the 02 Site Hopping Process
3.3.3 Hydrogen Atom on Top of Rest Atom
3.4 Conclusions
Chapter 4. Pseudo-Vacancy Diffusion on Si(lll)-7x7 Surfaces
4.1 Introduction
4.2 Results and Discussion
4.2.1 Dynamic Behavior of Pseudo-Vacancy
4.2.2 Identification of Pseudo-Vacancy
4.3 Conclusions
References